Travellers are facing major disruption on rail services between London Paddington and Heathrow Airport and Reading.
Damage to overhead electric wires is causing disruption to train journeys which is expected to last until noon, National Rail said.
It warned that passengers could face delays of up to an hour amid possible train cancellations and revised timetables.
Great Western Railway urged passengers to check before they travelled because some lines are blocked along the London Paddington to Reading route.
There are also reduced services on parts of the Elizabeth Line and the Heathrow Express between Paddington and Heathrow Terminal 5.
It comes as Transport for London said that Tube strikes will affect its services between Sunday and Friday, adding that “there is expected to be severe disruption across the whole Tube network”.
The strike by London Underground workers started on Friday after last-ditch talks failed to resolve a pay dispute.
The walkout by members of the Rail, Maritime and Transport union (RMT) comes during a dispute over a 5% pay offer and other issues including travel facilities.
Control staff are being 24 hours of action on Sunday, while from Monday to Wednesday nearly 10,000 RMT members will strike across the Tube.
